1. Field of the Invention
The present invention relates to a high temperature superconductor (HTS), in particular to a high temperature superconductor known as coated conductor as well as to a process for the production of a coated conductor resulting in improved orientation of the superconductor layer.
2. Description of Related Art
Coated conductors, which are also referred to as “second generation superconductors”, are useful in the production of long lengths flexible HTS tapes or wires. They are composed of a multilayer structure with a substrate, a superconductor layer and, typically, one or more buffer layer(s) between the substrate and the superconductor layer. The buffer layer(s) serve to compensate for the various different properties of the materials used. Finally, onto the superconductor layer, according to need, further layers such as metallic protection layer, may be deposited to complete the whole conductor structure.
High temperature superconductors, such as coated conductors, are promising candidates for a plurality of applications for power transmission cables, rotor coils of motors and generators, and windings of transformers, fault current limiters as well as of magnets, for example, for medical magnetic resonance imaging (MRI).
Though not restricted thereto, currently, the rare earth barium cuprate-type superconductors of the general formula REBa2Cu3O7−x (REBCO) are conventionally used in the production of coated conductors. A particular member thereof is that one known by reference YBCO-123 wherein the numerical combination 123 stands for the stoichiometric ratio of the metals Y, Ba and Cu.
A major problem in the production of coated conductors is the crystallographic orientation of the crystal grains of the superconductor material. In order to have good superconducting performance, for example in terms of critical current density (Jc) and critical current (Ic), the superconductor material must have a high degree of orientation or texture with the individual crystal grains being oriented essentially in parallel to each other and with an inclination against each other as small as possible.
The term “texture” relates to the distribution of crystallographic orientations of the crystal grains of a polycrystalline sample. A sample in which these orientations are fully random is said to have no texture.
Preferably, the superconductor layer has a biaxial texture with the crystal grains being aligned both in the same direction with respect to the surface plane (in-plane or a-b alignment) and perpendicular to the plane (out-of-plane or c-axis alignment).
The necessity of biaxial texture of high temperature superconductors stems from the fact that the grain boundaries strongly suppress or even block the critical current flow in the high temperature superconductor materials. The achievement of a high, almost single-crystalline perfection of the layers in long lengths, such as of kilometer lengths, as required for example, in cable production, is a major challenge in the production of coated conductors.
For the deposition of the buffer layers several techniques are known. These can generally be divided into two categories: vapor deposition techniques, such as vacuum techniques, and chemical solution deposition (CSD) techniques.
The principal difference is that in the vacuum techniques the material is supplied to the surface of the growing layer in atomic portions and the quality of the layer to be formed can be rather well controlled. In particular, the biaxial texture can be even induced during vacuum deposition as is the case of such a method as ion-beam-assisted deposition (IBAD) or inclined substrate deposition (ISD). For almost all vacuum techniques such as pulsed laser deposition (PLD), metal organic chemical vapor deposition (MOCVD), reactive sputtering, etc. suitable condition for texture improvement during deposition can be found rather easily. However, these deposition techniques require vacuum equipment which is expensive. Further, these methods are often limited by deposition rate and compositional control as well as of high costs of precursor materials.
A. C. King et al “The progress made using the combustion chemical vapor deposition (CCVD) technique to fabricate YBa2Cu3O7−x coated conductors”, IEEE transactions on applied superconductivity, vol. 13, no. 2, June 2003 is directed to an non vacuum chemical vapor deposition technique in the production of coated conductor wherein the buffer layers are deposited by combustion chemical vapor deposition (CCVD). Disclosed is a layer architecture for a coated conductor comprising a biaxially textured metal substrate, a ceria (CeO2) seed layer and a lanthanum zirconate (LZO) buffer layer deposited thereon, wherein both the CeO2 and LZO layer are each obtained by combustion chemical vapor deposition.
CCVD is a true vapor deposition process, wherein the substrate is coated by drawing it over a flame plasma obtained from vaporization of the precursors dissolved in a solvent. In CCVD the vaporized precursors need activation in a combustion flame before they reach the substrate surface. In this process the precursor solvent requires vaporization, activation and coating by drawing the substrate through the flame plasma. Furthermore, it is stated that in case of buffer layer deposition, a localized reducing atmosphere is used to prevent oxidation of the substrate metal which requires seals for allowing continued substrate passage between the open air and the localized reducing atmosphere. The locally reducing atmosphere is also necessary in the deposition of YBCO to minimize oxidation of the buffered substrates. Though vacuum technique can be avoided a complex apparatus is needed in view of the above requirements.
In the CSD technology a metal organic (MO) precursor layer of finite thickness is first deposited on a textured metallic substrate. After decomposition of the organic components in the layer, the compound of interest then grows in an usually amorphous matrix. In CSD orientation of layers is achieved by epitaxial growth onto a textured template. That is, a suitably textured substrate serves as a template for inducing the desired orientation in the layer to be grown. Similar, each further layer adopt the crystal orientation of the underlying layer onto which said further layer is grown.
A typical example of a CSD technique is referred to metal organic deposition or decomposition (MOD) and modifications thereof such as the sol-gel process or YBCO trifluoroacetate (TFA) process.
Chemical solution deposition does not require vacuum equipment. Further, the precursors used in chemical solution deposition are comparatively cheap. Thus, deposition of all the buffer layers required in the production of coated conductors and, preferably, inclusive deposition of the HTS layer by chemical solution deposition offer a low cost process suitable for commercial application. The principal difficulty for implementing this all-CSD technology is the fact, that the texture of the layer grown is usually not better than that of the substrate.
With the CSD technology highly oriented layers can be obtained when grown on single crystal substrates, which possess a high texture by nature. However, single crystals are very expensive and, further, are not available in the large area and long length as required for cable or wire application.
There are known textured substrates which are obtained by mechanical deformation of metal tapes followed by recrystallization annealing. A typical example of such textured metallic substrates is known as RABiTS (rolling assisted biaxially textured substrate). On such textured metal substrates buffer layers with suitable texture can be grown, which, in turn, can serve as template for transferring the desired texture to a superconductor layer to be grown on the buffer layer.
However, RABIT substrates currently available on the market are polycrystalline with a degree of misorientation of the grains which cannot be neglected. Moreover, when using CSD techniques the texture of the layer to be grown is often worse than that of the underlying template such as the substrate. For example, in the particular case of La2Zr2O7 (LZO) buffers, the out-of-plane texture is usually 15 to 30% inferior than that of the underlying Ni—W alloy RABITS. If such misorientation of the grain boundaries is transferred to the HTS layer to be grown, many grain boundaries in the final HTS layer are not transparent to the supercurrent.
Degradation of texture may make the network of such “bad” grain boundaries in the final HTS layer almost close to the percolation limit when the probability of the current breaking paths along the tape width becomes close to 100%.
Currently, this problem is circumvented by coated conductor manufacturers that utilize a CSD (MOD-TFA)-technique for depositing the HTS layer, by depositing all or some buffer layers via vacuum techniques, so that the texture of the buffer layers is not deteriorated or can be even improved compared to the metallic substrate.
However in view of costs, there is the desire for a buffer layer architecture using biaxially textured substrates and which is fully obtainable by CSD techniques (all-CSD).